1. Field of the Invention
The present invention relates to a control system for controlling a magnetic disk drive unit such as a hard disk drive unit, a floppy disk drive unit or the like. More particularly, the present invention relates to an improvement in a control system which is a computer system or the like comprising a disk controller operable to provide a function or clock signal for reading or writing data by a magnetic disk drive unit.
2. Description of the Prior Art
A computer operating as a control system to which a magnetic disk drive unit is connected, comprises generally a disk controller. The magnetic disk drive unit in such a system is controlled by the disk controller, which operates as a function of a clock signal for reading or writing data by the magnetic disk drive unit. In case of a computer system having a high speed data processing capacity of a higher degree than a medium level, a disk controller has a direct memory access (DMA) transfer function, by which data are transferred between a main storage device on the side of the computer system, namely on the host side and the disk controller when writing data into the magnetic disk drive unit or reading data from the magnetic disk drive unit. However, during the DMA transfer between the main storage device and the disk controller, a central processing unit (CPU) on the host side is in the hold state and the disk controller operates independently of the control of the CPU. On the other hand, the disk controller operates as a function of a clock signal for reading or writing data by the magnetic disk drive unit as described above. Accordingly, if a supply of the clock signal is stopped for some reason, the system remains in the state of a DMA transfer, causing a fault in the system.
In addition, generally, ir a supply of the clock signal is stopped during the writing of data into the disk by the magnetic disk drive unit, the internal operation of the disk controller consequently does not proceed and a write signal keeps being supplied from the disk controller to the magnetic disk drive unit. Accordingly, the magnetic disk drive unit remains in a write operation state, whereby erroneous data are successively written into the disk over a plurality of sectors, causing the data to be damaged.